The compressive load carrying capacity of a layer of resilient material may be increased several hundred percent by incorporating a plurality of spaced, parallel laminae fabricated of nonextensible material and oriented generally perpendicular to the direction of the anticipated compressive load. The laminae increase the compressive load carrying capacity of the resilient material by reducing the ability of the material to deflect or bulge in directions transverse to the direction of the compressive load. At the same time, the ability of the material to yield in shear or torsion in directions parallel to the laminations or transverse to the direction of the compressive load is substantially unaffected. The characteristics of such a laminated resilient material have resulted in the commercial acceptance for a variety of applications of bearings incorporating the material. One area of particular importance is the mounting of helicopter rotor blades on an associated rotor hub.
In a typical mounting system for the blades of a helicopter rotor, as shown for example in Rybicki et al U.S. Pat. No. 3,829,239, each rotor blade is secured to a rotor hub by two serially arranged laminated bearings. One laminated bearing, which has annular, disc-shaped laminations, resists the centrifugal load on the rotor blade and permits oscillatory pitch-change movements of the blade about its longitudinal axis. The other bearing, which has annular, spherically-shaped laminations, also resists the centrifugal load on the rotor blade and accommodates pitch-change, flapping and lead-lag movements of the blade. Since the pitch-change rotations of the blade are of a relatively large magnitude (i.e. 10.degree. to 15.degree. in each rotational direction), particularly when compared to the lead-lag and flapping motions, the combined torsional movement capabilities of the two laminated bearings must be relatively large. For economic and space reasons, the bearing having disc-shaped laminations is conventionally designed to accommodate a greater proportion of the torsional motion. As a result, the disc-type or thrust bearing is relatively long or tall. With increasing length or height, however, the bearing becomes increasingly unstable in a lateral or radial direction.
The lateral instability associated with a tall or long laminated bearing has been recognized and various proposals have been made to counter the instability. One such proposal is to modify the disc-like configuration of the laminations in the bearing so as to resist the lateral movements of individual laminations which result in buckling of the bearing. Representative configurations providing lateral stability are described and illustrated in Hinks et al U.S. Pat. No. 3,083,065 and Peterson U.S. Pat. No. 3,292,711. Bearings having laminations configured to resemble the laminations of the bearing shown in FIGS. 2 and 6 of the Peterson 3,292,711 patent have been utilized successfully in helicopter rotor blade retention systems. Nonetheless, as technology in the construction of helicopters has advanced, increased demands, in terms of higher centrifugal loads and prolonged service life, have been made on laminated bearings utilized in helicopters. Laminated bearings having laminations shaped generally as shown in the Peterson 3,292,711 patent, and, more specifically, as shown in FIGS. 2 and 3 of Johnson U.S. Pat. No. 3,807,896 and in FIGS. 1(b) and 3 of an article entitled "Elastomeric Bearing Application to Helicopter Tail Rotor Designs", authored by C. H. Fagan and appearing in the Journal of the American Helicopter Society, Volume 13, No. 4 (October 1968), have been unable to satisfy increased service life requirements. Such bearings have characteristically failed through failure and extrusion of the elastomeric laminations adjacent to each end of the bearing.
As more fully discussed in copending application Ser. No. 632,423, filed Nov. 17, 1975, now U.S. Pat. No. 4,040,690, it has been found that when large compressive loads are applied to a laminated bearing in which the layers are contoured to provide lateral stability, the contour of the layers may contribute to unusually large strains in the elastomeric layers adjacent an end of the bearing toward which all or a part of each layer projects. The excessive strains tend to cause early fatigue failure of the layers and to reduce the service life of the bearing. To accommodate the high stresses occurring in the elastomeric layers without introducing excessive strains, the layers of elastomeric material in a central lengthwise portion of a bearing according to the invention of application Ser. No. 632,423 have smaller compression moduli than the layers of elastomeric material adjacent the end of the bearing toward which the layers project. While the compressive stresses on the elastomeric layers are not reduced, the high compression-induced strains are substantially reduced and the life of the bearing is correspondingly increased. In a preferred embodiment, the elastomeric layers along the length of the bearing between the central lengthwise portion of the bearing and the end of the bearing toward which the layers project have progressively increasing compression moduli with increasing distance from the central portion of the bearing.
The invention of application Ser. No. 632,423 largely overcomes a significant problem that previously limited the fatigue life of certain laminated elastomeric bearings. The gradation or variation of compression moduli from one elastomeric layer to another in a laminated bearing not only reduces excessive strains in critical elastomeric layers, it can also minimize or eliminate all variations in compression induced shear strains at corresponding points on successive layers. The more uniform the compression induced shear strains become, the less likely it is that any elastomeric layer will fail in fatigue significantly sooner than the other elastomeric layers. By the same token, the more uniform the compression induced shear strains are within each layer, the less likely it is that any portion of any elastomeric layer will fail in fatigue before any other portion of the layer. Absolute uniformity of compression induced shear strains within a layer of elastomer is impossible to achieve because the strains necessarily decrease from some finite value adjacent each unconfined edge of the layer to zero at some point in the interior of the layer. Nonetheless, a more uniform wearing and fatiguing of each layer can be achieved by causing the compression induced shear strains adjacent the edges of the layer, which are normally the largest strains in the layer, to become more nearly equal to one another. The invention that is defined by the claims of application Ser. No. 632,423 is not directed to reducing variations between the compression induced shear strains developed at various points within a layer of elastomer. The application does recognize, however, that the compression induced shear strains adjacent the inner and outer circumferences of an annular elastomeric layer, for example, will often be significantly different. The difference in strains is recognized at page 10, lines 5 to 21 and at page 17, line 27 to page 18, line 19 of the application. At page 18, lines 17 to 19, a proposal is made to grade or vary the modulus of elasticity of an elastomeric layer in a radial direction within the layer. Although the proposal is made with respect to only one particularly configured laminated bearing, the proposal has more general applicability in terms of minimizing the variations in compression induced shear strain within an elastomeric layer or lamination of a laminated bearing.
Gradation or variation of the modulus of elasticity within an elastomeric layer of a laminated bearing is not unknown. Dolling et al. U.S. Pat. No. 3,941,433 describes and illustrates a laminated elastomeric bearing in which each annular elastomeric layer or lamination has a central annular portion of soft or low modulus elastomer. Two annular edge or peripheral portions of each layer are fabricated of stiffer or higher modulus elastomer in order to contain the softer central portion of the layer. The patent does not identify the problem of different compression induced strains within a layer of elastomer, nor does the gradation of the layers of the Dolling et al bearing offer a solution to the problem. Boschi U.S. Pat. No. 2,560,627 describes and illustrates a laminated elastomeric support that incorporates a lateral gradation of stiffnesses in each elastomeric layer similar to the gradation used in the Dolling et al bearing. The Boschi support does not include nonextensible laminations, however, and is only intended to accommodate relative displacements of opposite sides of the bearing. An arguably related gradation is found in the laminated bearings described and illustrated in Hinks U.S. Pat. No. 3,071,422 and Krotz U.S. Pat. No. 3,179,400, the latter of which is owned by the assignee of the present application. In the Krotz bearing, and in the bearing shown in FIG. 3 of Hinks, the thickness of each annular elastomeric layer continuously increases in a radial direction toward the outer circumference of the layer. Assuming that each layer of elastomer has a uniform modulus of elasticity throughout, the increase in layer thickness will result in a continuous decrease in compression modulus. In the Krotz bearing, at least, the purpose f the gradation of the layer thicknesses is to maintain throughout each layer a uniform torsional motion induced shear stress. Neither the Krotz patent nor the Hinks patent recognizes the problem of compression induced shear strain.